Earlier experiments by William C. Rose demonstrated that an oral source of histidine was not needed for the maintenance of nitrogen equilibrium in young, adult human subjects. Recently other authors have questioned the conclusion that histidine was a non-essential amino for adult men. Therefore a rigorous experiment was designed to search for answers to these questions. In the present 72-day experiment, an adult human was maintained for 48 days with the sole source of nitrogen being the amino acids in the intravenous alimentation solution (10.0 g N and 2600 calories per day). A 3-day lysine-free intake caused a marked negative nitrogen balance. Upon the deletion of histidine for 27 days, apparent positive nitrogen balance, the partition of urinary total nitrogen, and body weight continued essentially unchange throughout the period. The serum total protein, albumin, prothrombin, alkaline phosphatase, lactic dehydrogenase, urea N and creatinine were constant for the 72 days. Many other analyses and tests of organ systems were performed. These and other unpublished findings must be considered in the evaluation of the essentially of histidine for normal man.